Create README.md files in each subdirectory

- Contents from webpage
- Code that creates and runs tests now also allows non-R files in `libs` and `tests` subdirectories
- README file in main directory is now also formatted as Markdown, with the project name as title
- Default files are stored as plain files and not in a tar archive anymore

Fixes #128.

See e.g. https://github.com/johnmyleswhite/ProjectTemplate/tree/128-readme/inst/defaults/full/cache for an example how this looks on GitHub.